(1) Field of the Invention
This invention relates to the formation of electron spin EPR pairs, i.e. fully spin entangled electrons, and manipulation thereof. The invention also relates to the formation and interconversion of flying and static electron spin qubits.
(2) Description of the Art
Quantum information systems process or transmit information using quantum bits, referred to as qubits. Often it is desired that the qubits are formed as entangled EPR (Einstein-Podolsky-Rosen) pairs. As the skilled person will be fully aware EPR particle pairs are particles such as electrons or photons having fully entangled quantum properties that may be spatially separated. A qubit entangler is therefore of fundamental importance in quantum information systems. Having formed an entangled qubit pair it is also necessary to be able to manipulate the qubits. The ability to interconvert static and flying qubits is a basic requirement for a functional system. In the art the term “static” is used to refer to a qubit held in place and the term “flying” is used to refer to a qubit which is moving. Interconversion is the process of turning a static qubit into a flying qubit or vice versa.
In the solid state both the creation of an entangler for massive particles and the interconversion of qubits are hard tasks since the process of generation and detection must take place in a controlled way and in a much shorter time than typical decoherence times.
Any two-state quantum system has the potential to act as a qubit. One promising candidate is the spin of a single electron, which has two basic states, spin-up and spin-down. Electron spins have a number of possible advantages for use as qubits such as relatively long decoherence times, manipulation by magnetic fields, realisation in solid state devices, ability to be transported from one location to another and potential for controlled interaction with other spins to exchange quantum information.
Various entangler schemes have been proposed for generating electron spin EPR pairs. A number of approaches have used one or more quantum dots in which electron spins are delocalised by lowering barriers, allowing them to escape or be channeled in certain directions. For instance Saraga et al. “Coulomb Scattering in a 2D Interacting Electron Gas and Production of EPR Pairs” Phys. Rev. Lett. 92, 246803 teaches coulomb scattering to produce singlet states with frequency selection of the interacted electrons. Zhang et al. “Generation of Spatially Separated Spin Entanglement in a Triple Quantum Dot System”, Phys. Rev. A, 69, 042307 discusses use of a triple quantum dot structure and adiabatic manipulation of gate voltages. Recher et al. “Andreev tunneling, Coulomb blockade, and resonant transport of nonlocal spin-entangled electrons”, Phys. Rev. B., 63, 165314 propose a spin entangler based on an s-wave superconductor coupled to two quantum dots. Hu et al. “Double quantum dot turnstile as an electron spin entangler”, Phys. Rev. B, 69, 115312 study use of a double dot turnstile as a reliable entangler. All of these approaches have yet to be verified experimentally and have issues associated with control of the qubits and decoherence.
Another suggested approach is to utilise the interaction between electrons carried by surface acoustic waves (SAWs). The spin of a single electron in a minimum of the electrostatic wave produced by a SAW in a semiconductor quantum wire will interact with the spin of a single electron in a neighbouring quantum wire if they are sufficiently close and it has been suggested this interaction will produce entanglement between the two electrons. See, for example, Gumbs et al., “Quantum entanglement for acoustic spintronics”, Phys. Rev. A. 70, 050302 or Ebbecke et al. “Acoustoelectric current transport through a double quantum dot”, Phys. Rev. B, 72, 121311. The main disadvantage of using such a scheme, and related schemes using propagating and bound spins, is that it is very difficult to control the degree of entanglement and, in particular, full entanglement is extremely difficult to achieve in a controlled way due to the exponential change in exchange coupling with distance.